Bottled water may not contain all the minerals your fish needs to stay healthy. Conditioned tapwater is recommended. And Prime by Seachem is the most recommended conditioner because it detoxifies ammonia.
The cloudiness is a harmless bacteria lbloom. Unfortunately, it's not the nitrifying bacteria you want to cultivate in your tank.
Do not use salt.
A 50% change every three days is appropriate for a 2.5g tank. Use Prime at 2-drops/gal of tank size with every change.
A cycled tank is healthier, and a 2.5g can be safely cycled with a sponge filter (recommended). But a small tank is a little finicky. I'd say get used to tank maintenance and fishkeeping for a month or so. Read up on cycling---several stickies in this section. When you're comfortable caring for your fish, we'll coach you through the cycling
Live plants are a definite plus if you can handle them. And as many as you can maintain. Makes it a lot safer, especially in a small tank. See the "Planted tank" section of this forum.
Welcome to the forum.